Climate change is one of today's most serious environmental challenges. The increasing carbon dioxide concentration in the atmosphere is largely the cause of global warming. CO2 of human origin is essentially emitted into the atmosphere by the combustion of fossil fuels in thermal power plants or is produced by cement plants or steel plants.
To combat the CO2 emissions, one technology is designed to capture the CO2 emitted during the combustion of carbon-containing fuels to transport it and/or to sequester it underground.
It should be observed that the capture of the CO2 from a stream also containing nitrogen, oxygen, argon, hydrogen, methane and/or carbon monoxide, causes the stream to be enriched with these products. The CO2-depleted stream can then be used in a neighbouring process or can be recycled to the process that produced it. The method for producing a CO2-enriched gas can therefore also be seen as a method for deballasting CO2 from the gas to be treated. These two automatically linked functions can be exploited simultaneously. For example, recycling the CO2-deballasted gas to the blast furnace serves to utilize the CO and the hydrogen while the CO2-rich fraction can be sequestered.
In all cases, the CO2 problem will require extracting at least part of the CO2 contained in various gases produced by industry. Many methods will be used to capture this CO2. One of the methods is adsorption. The CO2 can be trapped at high temperature, that is above 150° C., or, on the contrary, at about ambient temperature, the CO2-containing gas then preferably being at a temperature below 60° C.
The adsorption unit may be of the PSA type.
When regeneration takes place by lowering the pressure, this involves a PSA (Pressure Swing Adsorption) process; PSA process means actual PSA processes, that is with the adsorption phase taking place at a pressure substantially higher than atmospheric pressure, VSA (Vacuum Swing Adsorption) processes, in which the adsorption phase takes place at about atmospheric pressure with regeneration under vacuum, VPSA and similar processes (MPSA, MSA, etc.) with an adsorption phase taking place under a few bar and regeneration under vacuum. This category also includes systems which are regenerated by flushing with a purge gas, a gas which may be extraneous to the process itself. In this case, the partial pressure of the impurities is actually lowered, thereby allowing their desorption. The acronym PSA is used below for any one of these units.
PSA and VPSA units (that is with adsorption at medium pressure, generally between 2 and 10 bar abs, and regeneration under moderate vacuum, generally above 250 millibar absolute, preferably about 350 to 500 millibar absolute) have already been investigated extensively for various types of separation: production of high purity hydrogen, oxygen and/or nitrogen from air, methane from a CH4/CO2 mixture, CO from syngas, etc. These PSAs are constructed from well-known elementary steps: adsorption step, balancing steps, purge providing, blow-down, purge, repressurization, rinse.
These steps can be sequential or some may be simultaneous. One can for example consider the description of the cycles used in a number of PSA processes relative to various applications, cycles which can be easily adapted to at least partial capture of the CO2:                EP 1 004 343 describes a cycle initially developed for H2 PSA with two regeneration pressure levels, with 4 adsorbers and one balancing;        EP 1 095 689 describes a cycle with 2 adsorbers developed for the production of oxygen from air, a cycle comprising a repressurization with the unadsorbed gas, one balancing, a final repressurization with the feed gas, a production step, a blow-down step partly using a vacuum pump, and a purge phase;        U.S. Pat. No. 4,840,647 describes a cycle with 2 adsorbers, more particularly adapted to the capture of an easily adsorbable component such as CO2;        EP 1 023 934 describes a H2 PSA cycle with recycle of part of the low pressure waste gas to the gas to be treated;        U.S. Pat. No. 6,287,366 describes a O2 VSA cycle illustrating the combined steps such as simultaneous blow-down via the 2 sides of the adsorber, repressurization with two different fluids, etc.        
Most of the cycles described in the literature are directed towards the production of the least adsorbable gas or gases, the more adsorbable gases constituting the waste gas. This type of cycle can nevertheless be used to capture CO2. In this case, the PSA is for example regulated to the CO2 content in the light gases. In fact, producing a CO2-enriched fraction at low pressure is equivalent to producing a CO2-depleted fluid at the adsorption pressure.
US 2007/0261551 relative to CH4/CO2 separation provides an example of a H2 PSA cycle with a high pressure adsorption phase, 2 balancings, one co-current blow-down with purge providing, a final blow-down, a low pressure purge step with the gas previously recovered and the product gas, and a final repressurization with the feed gas and the product gas.
This type of cycle can optionally be improved by the addition of steps more specific to the production of the most adsorbable gas, that is the CO2 here. These additional steps are essentially steps of recycling part of the gas issuing from the blow-down, recycling to the feed or directly to another adsorber. In the latter case, this is referred to as a rinse step. Recycling to the feed generally consists in tapping off the least CO2-rich fraction(s) of the gas issuing from the PSA from the counter-current blow-down or purge steps, in order to obtain a waste gas richer in CO2. In this way, fewer of the lightest components (hydrogen, CO, methane, etc.) are obviously lost in the CO2-enriched waste gas, and they can be used in another unit.
U.S. Pat. No. 4,077,779 describes a cycle with 4 or 6 adsorbers comprising the recycling of part of the blow-down gas to carry out a step of recycling to the feed or directly to another adsorber, as well as a purge step with a gas extraneous to the PSA. It is stated that this cycle can be used both for hydrogen production and for methane/CO2 separation. The literature also describes cycles intended to extract CO from a syngas. In this case, the CO is the most easily adsorbable gas on an adsorbent that is specific to it. This type of cycle is directly transposable to the capture of the CO2 contained in essentially less adsorbable gases after adsorbent replacement.
CO2 sequestration is employed primarily on units producing large flows of CO2-rich gas. Among them, mention can be made of the waste gases from carbon-containing fuel-fired electric power plants, in particular oxy-fuel combustion, cement plant gases, gases produced by steelmaking processes, or even syngases obtained by partial oxidation or steam reforming of carbon-containing fuels. In addition to their CO2 content, these gases have the common feature of containing water vapour and dust. It is customary to remove most of these dust by methods well known to the profession: electrostatic filter, water scrubbing, venturi, cyclone, static cartridge or bag filter, dynamic filter regenerable in operation (by reverse gas flush-pulse), isolable filters mounted in parallel with the possibility of regeneration or replacement, the main unit remaining in service. Use is not made of a total filter—that is a medium that only allows elements having the size of gas molecules to pass through—and it is conventional that after filtration, these gases contain residual dust which raise no particular problem because generally vented to the air, burned or recycled without other treatment. More precisely, this generally involves gas streams containing less than 50 to 100 mg per m3 of solid matter, even more generally less than 20 mg per m3 (for greater clarity, m3—here and below—means m3 of gas relative to 0° C. and atmospheric pressure, although in the profession, it is often a matter of real m3, for which the pressure and temperature conditions should be indicated every time).
The quantity of dust contained in the gas and the particle size distribution can be obtained by any one of the known methods which are not described here. Below, when speaking of particle size distribution, expressed in microns, reference is made to the main dimension of the particle (length for an elongate cylinder, diameter of the circumscribed sphere for an essentially spherical or cubic particle). The percentages indicated concern the number of particles having a particle size lower or higher than a given value.
It has appeared that the fine residual dust—that is after filtration or any primary trapping—which, in the absence of liquid water, can pass through the various components of the PSA unit without any particular problem, being easily transported by the gas, tends to cake and deposit in the presence of moisture. This has been observed and interpreted on an industrial unit successively having water saturation zones and water unsaturation zones due to the process employed. Deposits were systematically observed in the presence of saturation, whereas there was no accumulation of dust in the non-condensation zone. Due to the presence of filters, it was found that the quantity of particles per m3 or the particle size distribution was not the predominant factor for the presence or absence of deposits, but clearly the presence of moisture (liquid water).
In fact, when such a gas, that is to say simultaneously containing water vapour, dust and a large quantity of CO2, is treated by an adsorption unit, in particular in a PSA, it undergoes strong thermal effects due to the adsorption-desorption of the CO2. Although the gas enters the adsorber at ambient temperature and at—or above—its condensation point, cold spots are created in the adsorber during the generation steps with passage below the water condensation point.
It is clear that these problems are aggravated by the fact that, in each cycle, there may be periods of condensation with caking of the fines followed by periods of drying, for example in the adsorption phase, which thereby cause the successive dust deposits to adhere to their support.
These caked deposits can ultimately cover the adsorbent particles or at least block their pores, clog the various instrumentation connections, dwell in the essential equipment, including the valves in particular. A loss of tightness of the latter causes poor operation of the PSA, particularly a drop in performance or even a blockage of the system. The same applies if the adsorbent is partly damaged.
In the case of CO2 VSA, the thermal effects may be less severe in the adsorbent beds themselves due to the weaker pressure effect than in a PSA, but condensation is probable in the vacuum pump if the CO2-rich gas is repressurized.
On this subject, the document of Gang Li et al, “Capture of CO2 from high humidity flue gas by vacuum swing adsorption with zeolite 13X”, Adsorption (2008) 14: 415-422, addresses the problem of the creation of cold spots, and of water condensation in the vacuum pump.
In fact, when a PSA unit is used, it is common to treat the wet gas directly in said unit without prior drying. Various specific adsorbents for stopping the water can be used for this purpose.
For example, the document by Ralph Yang, “Gas separation by adsorption processes”, Butterworth Publishers 1987, teaches in particular that water vapour is adsorbed very strongly on zeolites and raises serious problems when zeolites have to be used to carry out a separation such as the separation of air gases, so that the tendency is to use silica gel or activated alumina to stop the water, and to integrate this stripping with the main stripping that uses zeolite. This naturally gives rise to the use of multibeds in the same adsorber, each layer of adsorbent being dedicated to stopping an impurity.
Similarly, N2 PSA, H2 PSA and PSAs supplying instrument air directly treat a wet feed gas. This is also the case of PSAs producing CO2 and simultaneously hydrogen relatively similar to the PSAs mentioned here. On this subject, details can be found in Douglas Ruthven et al., “Pressure Swing Adsorption”, which teaches that the water is stopped by a first series of adsorbent beds.
In general, without any specific problem such as dust, a person skilled in the art will not install TSA and PSA adsorption units in series, but will adapt his process and the adsorbents to carry out the intended separation in a single PSA unit. In particular, the problem of the presence of water in the feed gas is well known and PSA type solutions operating under such conditions are also well known today, as shown in the various examples given above.
To remedy the problems of dust deposition in the presence of water, a total filtration can be installed upstream of the PSA unit. In case of very large throughputs, generally at low pressure, the filtration surface area to be installed must be enormous and the same applies to the investment.
As an alternative, the scrubbing techniques and the number of scrubbing stages can be improved to remove all of the residual dust or at least to obtain residual contents which are sufficiently low for their caking during one or more years in or on the equipment to incur no drawback. A simple calculation of the quantities of dust entering the system, considering the large treated throughputs mentioned above—leads to acceptable residual values lower than about 10 mg per Nm3 of gas to be treated, preferably about one microgram per Nm3. These two techniques can be used in series. The direct costs (investment) and indirect costs (pressure drop on the gas, pumping energy, etc.) then become very high.
Another solution is to operate the adsorption unit at a sufficiently high temperature to remain above the condensation point in all circumstances, both in the adsorber and in the ancillaries. With the conventional adsorbents mentioned above, the adsorption capacity is reduced too much by raising the temperature, and such units would not be efficient. Vacuum pumping of hot gas would also be very costly in terms of investment and energy. These solutions are nevertheless under investigation but are not conclusive for the time being.
On this basis, a problem that arises is to propose a method for producing a CO2-enriched gas employing a PSA unit, in which the caking of the solid particles in the PSA unit is reduced.